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Transcript
CHAPTER 23 THE EVOLUTION OF POPULATIONS I. Student misconceptions 1. Students often misunderstand the significance of individuals and individual variation to the theory of evolution by natural selection. a. The term adaptation is used to describe changes in an individual over its lifetime (such as physiological adaptation). It is also used to describe changes in traits that increase the evolutionary fitness of an organism in its environment. The dual use of this key term may confuse students. b. Students mistakenly think that evolutionary change comes about as traits gradually change in all members of a population, rather than realizing that individuals with favorable heritable traits come to make up an increasing proportion of the population. 2. Many students find it hard to understand the Hardy-Weinberg principle and do not know how and when to use the Hardy-Weinberg equations. Students ask why the principle is important, since the assumptions are never fully met in natural populations. Emphasize to students that the Hardy-Weinberg principle clarifies the factors that alter allele frequency, and that it does not imply that allele frequencies are static. Explain to students that the Hardy-Weinberg equations are used with respect to a specific gene. When using the equations, be very careful to distinguish between allele and genotype frequencies. 3. Students may be confused about the role of chance in evolution and natural selection. New alleles arise by chance mutations, new combinations of alleles arise by the shuffling of genes in sexual recombination, and chance events may alter allele frequencies in small populations. Certainly, chance is important in evolutionary change. However, students may think that evolution itself proceeds by an accumulation of changes occurring by chance. Such students completely misunderstand the role of natural selection as the mechanism of adaptive evolution. Genetic variation does arise by chance. However, the action of natural selection to favor variants that survive and reproduce with relatively high success in their environment is not based on chance. 4. Students do not fully understand the role of the environment as an agent of natural selection. They may think that the environment acts on populations by causing the appearance of favorable traits, rather than recognizing that the environment affects the survival of individuals that display favorable traits. Many students do not recognize that as the environment changes, the advantages and disadvantages of specific traits also change. Therefore, natural selection has different effects/products in different environments. Students do not necessarily recognize that other organisms—competitors, Student Misconceptions for Campbell Biology, 9th Edition, © Pearson Education, Inc. 23-1 predators, prey, parasites, and mutualistic symbionts—are an important component of an organism’s environment. II. Pre-test to identify student misconceptions prior to addressing the material covered in Chapter 23 Read the following statements about the role of chance in evolution. Identify each statement as TRUE or FALSE. 1. The ultimate source of new alleles is mutation, random changes in the nucleotide sequences of an organism’s DNA. True 2. Evolutionary change proceeds by an accumulation of changes that occur by chance. False 3. An allele may be lost from small populations due to chance events unrelated to the fitness consequences of the allele. True III. IV. How can instructors address and correct the misconceptions that students have about the evolution of populations? 1. Use data from a real or imaginary nonhuman population to show how the Hardy-Weinberg equations may be used with respect to a specific gene, in order to determine whether or not the population is evolving with respect to this gene. 2. Alters and Nelson (2002) discuss the importance of a constructivist approach in teaching evolution. They emphasize that active learning strategies are far more effective than straightforward lectures in forcing students to confront and resolve their misconceptions about evolution and natural selection. Stern (2004) provides examples of good and poor questions about natural selection. Good questions require students to use what they know, rather than what they have memorized, to explain phenomena or solve problems. Instructors can use student responses to such questions to plan further instruction. 3. Some students may be resistant to learning about evolution because they think it is inconsistent with their religious beliefs. Rutledge and Warden (1999) developed MATE (Measure of the Acceptance of the Theory of Evolution), a 20-question Likert-scaled instrument, to measure high school teachers’ understanding and acceptance of evolutionary theory. This test may also be used as a pre-test or post-test in a general biology class. Though not developed for this purpose, it may give instructors a measure of the extent of their students’ resistance to instruction about the theory of evolution. Post-test to identify whether students have corrected their misconceptions 1. As discussed in the text, the fruit fly Drosophila melanogaster has an allele that confers resistance to DDT and similar insecticides. Laboratory strains of D. Student Misconceptions for Campbell Biology, 9th Edition, © Pearson Education, Inc. 23-2 melanogaster have been established from flies collected in the wild in the 1930s (before the widespread use of insecticides) and the 1960s (after 20 years of DDT use). Lab strains established in the 1930s have no alleles for DDT resistance. In lab strains established in the 1960s, the frequency of the DDT-resistance allele is 37%. Explain why each of the statements below is TRUE or FALSE. a. Some fruit flies evolved resistance to DDT in order to survive. False – This statement suggests that individuals intentionally evolve DDT resistance. b. The evolutionary fitness associated with the heritable trait of DDT resistance changed once DDT use became widespread. True c. Fruit flies became more resistant to DDT over time. False – More individuals have resistance to DDT; individuals do not become more resistant to DDT. d. When DDT was widely used, fruit flies with DDT resistance had greater evolutionary fitness than fruit flies lacking DDT resistance. True e. Alleles for DDT resistance may have been present but rare prior to DDT use. True – Alleles for DDT resistance may also have arisen by mutation during the period of pesticide use. f. Alleles for DDT resistance arose by mutation during the period of DDT use because of selection for pesticide resistance. False – This statement suggests that mutations occur in response to need. 2. V. Imagine that a population exists in which no mutations occur. Could such a population evolve? Explain. Discuss this question with your students, emphasizing the importance of mutation as the source of variation on which natural selection can act to produce evolutionary change. References Alters, B., and C. Nelson (2002). Perspective: Teaching evolution in higher education. Evolution, 56(10), 1891–1901. Rutledge, M., and M. Warden (1999). The development and validation of the measure of acceptance of the theory of evolution instrument. School Science and Mathematics, 99(1), 13–18. Stern, L. (2004). Effective assessment: Probing students’ understanding of natural selection. Journal of Biological Education, 39(1), 12–15. Student Misconceptions for Campbell Biology, 9th Edition, © Pearson Education, Inc. 23-3